Anchor Rode Bridle
An anchor bridle will help reduce sailing on the anchor rode and the resulting roll that occurs when the boat reaches the end of a swing and turns her beam to the swell. The bridle transfers the load to opposite cleat from a tack, thereby leveraging the boat back from that tack.
The bridle will also help avoid going beam to a high wind in a storm and the resulting strain on the ground tackle that can drag the anchor and place the boat in harms way. In conjunction with a small riding sail, the bridle has shown to reduce sailing on the rode.
My experience in twenty mph winds is that my riding sail reduced hunting by 40% with another 40% reduction using the bridle. On that occasion, the total reduction eliminated the roll that was occurring on offending tacks.
Probably the most recognized knot to accomplish this is the bowline on a bight using either of two variations, single or double. The sometimes called double bowline creates two loops where the single bowline on a bight creates one. Make sure the two loop lengths are such that the knot is located about a foot or so in front of the bow and that when formed they are equal length. Use two turns around the cleats to lock the loops onto the cleats. It should be noted that two anchor chocks are desirable though some boats will allow simply using forward cleats without the need for dual chocks.
Single bowline on the bight
The single loop bowline on a bight can also be knotted in the middle of a section of line and hence used to make a bridle at some point on the anchor rode. I use the single bight bowline and place the bight over and around the port cleat and bring the rode to the starboard cleat so as to center the knot ahead of the boat about a foot. This is the knot pictured.
Either knot will accomplish the same task.